The present invention relates to lasers, and more particularly to a long wavelength Neodymium Yttrium Aluminum Garnet (NdYAG) laser which may be operated in either a surgical cutting or coagulating mode.
The use of lasers in surgery and medicine is expanding very rapidly to the point where lasers have become important surgical and therapeutic tools. There are, for example, CO.sub.2 lasers used in surgical procedures to produce a scalpel-like incision, and NdYAG lasers, heretofore of little use for cutting, which are utilized to cauterize large blood vessels such as are involved in stomach hemorrhages or for destroying tumors in the bladder.
Medical applications for NdYAG lasers have been developed since there is a region of the light spectrum, in the red and near infrared, where human tissue is transparent. This region lies between the visible, where chromophores such as hemoglobin are strongly absorbing, and infrared, where water within tissue is absorbing, and is often referred to as a tissue "window". Light from the NdYAG laser which typically radiates at 1.06 micrometers is in the near infrared and falls within this window. When light from the NdYAG laser strikes most tissue it will be transmitted and scattered to a typical depth of about 1 centimeter before the intensity is too low to have therapeutic effect. If the laser is sufficiently powerful, deep in-situ heating of tissue is possible resulting in, for example, vessel shrinkage and associated cauterizing effects and deep coagulation necrosis.
Due to its cauterizing laser action at 1.06 micrometers, the NdYAG laser has found little use for surgical cutting but instead has been heretofore limited to the above noted types of therapeutic uses. Additionally, since the beam from an NdYAG laser can readily be coupled to and transmitted by a flexible fiber optic cable, the beam may be passed along a gastroscope or coupled through a cystoscope. The NdYAG laser can hence be easily used within the body without open surgery.
On the other hand, the CO.sub.2 laser radiates in a far infrared part of the spectrum which is strongly absorbed by tissue water. When this laser is focused onto tissue it causes tissue vaporization since the radiation is absorbed by water in the first few layers of cells which are heated to the point of explosive vaporization. The cellular layers are vaporized away and the focused beam "cuts" into the surface to make a scalpel-like incision until the beam is interrupted. The CO.sub.2 laser is used as a "blood free" knife since small vessels adjacent the incision are cauterized by the laser. Additionally, laser incisions appear to have unusual propensity for healing. For example, cold knife incision of skin tumors will result in ulceration, whereas laser vaporization of these tumors result in clean reepithelialization. Unfortunately, the CO.sub.2 laser will not propagate along flexible fiber optic cables so that beam delivery must be through an awkward combination of tubes and mirrors.
Research is presently being performed to develop CO.sub.2 laser fibers which would permit endoscopic laser surgery. However, because of the limited hemostatic property of the CO.sub.2 laser, possible complications arising from vessel performation during endoscopic CO.sub.2 laser surgery, has limited the usefulness of this research. Therefore it is desirable to develop ways to make a NdYAG laser beam, which will propagate along a quartz fiber, cut tissue. If it were possible to operate the NdYAG laser either in a cutting or coagulating mode, then endoscopic surgery could be performed without fear of complications from blood loss.
The wavelength of conventional NdYAG lasers falls at 1.06 micrometers in a region of very weak water absorption. The NdYAG laser, however, also typically has a transition around 1.32 micrometers and a very low gain laser transition around 1.44 micrometers. This latter wavelength coincides with a strong water absorption band, and light at 1.44 micrometer wavelength is absorbed by water after passing only about 0.3 millimeters into the water. In contrast the penetration distance of light into water is about 10 cm for light in the wavelength range of 1.32 micrometers and 100 cm for light in the wavelength range of 1.06 micrometers. Therefore, since animal tissues are comprised mainly of water, if a powerful focused laser beam at 1.44 micrometer wavelength were to strike tissue then it is possible that the tissue surface will heat and evaporate away in much the same way as a CO.sub.2 laser beam provides a scalpel-like incision as described above.
Unfortunately, Nd, which is suspended in a YAG crystal host and which is also the atom responsible for laser action from NdYAG, is absorbing in the spectral region of 1.44 micrometers. NdYAG is effectively 100% transparent in the wavelength regions of 1.06 micrometers where NdYAG has hitherto been used as a laser medium. However, laser oscillation around 1.44 micrometers is more difficult partly because self absorption of the Nd occurs at 1.44 micrometers due to an absorption on the electronic transition .sup.4 I.sub.9/2.fwdarw. .sup.4 I.sub.15/2, which is centered at 1.485 micrometers. Though this absorption may be minimized by using crystal material with Nd concentration as low as one half precent of the total NdYAG weight it is obviously not possible to eliminate self absorption in this way. Alternately self absorption by Nd at 1.44 micrometers could be reduced by cooling the laser crystal since self absorption only occurs because of thermal spreading of the absorption line. However, cryogenic cooling is impractical in most NdYAG applications.
Self absorption by a laser material is usually grounds for dismissing a potential laser transition. For the 1.44 micrometer line in NdYAG this is particularly true because the gain around 1.44 micrometers is very low and the 1.44 micrometer transition must compete for laser oscillation with the 1.06 micrometer transition which has typically several hundred times more gain. The NdYAG laser has thus tranditionally been considered a very unlikely candidate for high powered emission at 1.44 micrometers.
One report in the literature describes operation by using selective optics which suppress laser oscillation at 1.32 micrometers and 1.06 micrometers. However, the continuous output power reported from a 100 watt 1.06 micrometer laser was insignificantly small at 1.44 micrometers and was about 0.4 watts. The reason for this low output power is the very low gain or amplification of the laser at 1.44 micrometers so that in order to achieve laser oscillation the transmitting resonator mirror must be highly reflecting. The report describing laser action at 1.44 micrometers describes a transmitting mirror which transmits about 0.5% and reflects 99.5%. However, since the laser rod will typically absorb 10% of a 1.44 micrometer laser beam passing through it, each time light is reflected around the reasonator the laser beam loses 2 times 10% or 20% by absorption in the laser rod and only 0.5% by transmission through the output transmission mirror. As a consequence, the laser power is lost in absorption rather than passing through the output transmission mirror to form a useful laser beam.
It has been discovered, however, that it is possible to obtain a powerful beam of light from NdYAG laser at 1.44 micrometers wavelength if the laser mirrors are designed to prevent laser action from occurring at high gain laser transitions such as at 1.06 and 1.32 micrometers, and the laser drive or lamp is repetitively pulsed at high transient power rather than running the laser drive or lamp with continuous current. Under these two conditions, it is possible to obtain substantial average laser power from a NdYAG laser at 1.44 micrometers. The beam from such a laser, when focused onto tissue, will evaporate the tissue and result in a scalpel-like incision.
It has been discovered that under a repetitively pulsed drive lamp condition, it is possible to achieve laser oscillation with an output transmission mirror which is, for example, 20% transmitting and 80% reflecting. As a result, if laser rod absorption for each cycle of light is 20%, it has been found that the laser power is divided equally between the losses in the rod and as useful output power. It is believed that under conditions of intense pulsed lamp light the gain or amplification of light in the laser rod becomes very high and that laser oscillation will occur even though the losses from combined rod absorption and mirror transmission can be 50% for each cycle of light through the laser resonator. Additionally, it appears that oscillation at a wavelength of 1.06 and 1.32 micrometers will remain suppressed by laser resonator selectivity under these conditions. Thus, it is believed that the high peak power pulses of the laser drive permits laser oscillation at 1.44 micrometers with a relatively high transmission output mirror which reduces the detrimental effect of intrinsic absorption in the laser rod at 1.44 micrometers.
Thus, the combined use of a frequency selective laser cavity together with a repetitively pulsed high peak current lamp will result in an average laser power of at least 40 watts at 1.44 micrometers from a laser typically designed to produce 100 watts at 1.06 micrometers. The technique developed for producing the NdYAG laser cutting beam has been to shift the wavelength of laser radiation from 1.06 micrometers, where tissue is transparent, to 1.44 micrometers in which tissue water absorption is strong. As a result of shifting the wavelength of laser radiation from 1.06 micrometers to 1.44 micrometers, the penetration distance of the laser beam in tissue falls from about 1.0 centimeter to about 0.3 millimeters. Hence, a focused laser beam at 1.44 micrometers is absorbed in most tissues to produce a "slice" approximately 1/4 of a millimeter deep and the tissue at the focus vaporizes away in much the same manner as with the CO.sub.2 laser. The beam at 1.44 micrometers may thus be alternated with a beam at 1.06 micrometers by selectively mounting appropriate mirrors at the ends of the laser rod, and by providing appropriate circuitry to convert the pulsed lamp output to a continuous lamp output.
Additionally, even though the 1.44 micrometers NdYAG beam falls in the absorption band in quartz caused by trapped water, some quartz fiber is almost water free, and consequently, it is possible to transmit both a coagulating 1.06 micrometer beam and a cutting 1.44 micrometer beam through the same quartz fiber. Thus, the NdYAG laser of the present invention may be adapted to various types of endoscopes and operating microscopes for use in various surgical and therapeutic applications.